The disclosure relates to a method for reducing thermal conductivity in thermal barrier coatings and, more particularly, the disclosure relates to a method of reducing thermal conductivity of thermal barrier coatings by incorporating metal oxides into the ceramic matrix.
Gas turbine engines are well developed mechanisms for converting chemical potential energy, in the form of fuel, to thermal energy and then to mechanical energy for use in propelling aircraft, generating electrical power, pumping fluids, etc. At this time, the major available avenue for improved efficiency of gas turbine engines appears to be the use of higher operating temperatures. However, the metallic materials used in gas turbine engines components are currently very near the upper limits of their thermal stability. In the hottest portion of modern gas turbine engines, metallic materials are used at gas temperatures above their melting points. They survive because they are air cooled. But providing air cooling reduces engine efficiency.
Accordingly, there has been extensive development of thermal barrier coatings for use with cooled gas turbine aircraft hardware. By using a thermal barrier coating, the amount of cooling air required can be substantially reduced, thus providing a corresponding increase in efficiency.
One common thermal barrier coating (TBC) consists of a yttria stabilized zirconia ceramic known as 7YSZ. 7YSZ coatings typically exhibit thermal conductivity values of approximately 1 W/m° C. to 1.9 W/m° C., depending upon the process used to deposit the coating. It would be preferable to reduce this thermal conductivity by 50% or more without substantially increasing the mass of the coating. Because coatings are often applied to the airfoils of rotating parts, and assuming no substantial change is coating thickness occurs as a result, small increases in the density of the coating can result in large forces being applied to the rotating part. Therefore, an ideal coating would couple reduced thermal conductivity with reduced mass.